depending on what you got connected to you 300w, i guess any card will work... if it's a good power supply
as for the card, i would go for the 7600GT too
I guess this is a point to make. If all you have connected is a processor (depending on what you have), the vid card, a HD and a CD you'll be ok.
In my case, I have a P4 3.2gh, 4 512mb of ram, 5 hard drives on either SATA or gigaraid, a CD-RW, a DVD-RW, a removable HD rack with fan controler, a 4 position temp reader/display with 4 fan controler (cool drive 4), a 7800GS Overclocked spitting out to two monitors, and including the CPU and GPU I have 6 fans (2 of which are almost always off).
yeah, my rig would cry if I put a 300w on it.